1. Field of Invention
This invention relates to the field of damping torsional vibration. More specifically, this invention relates to a device for controlling vibration in a vehicle steering system using a magnetorheological damper.
2. Background
Automotive vehicle steering systems are subject to a large number of forces that may cause vibrations that are felt by the driver through the steering wheel. These undesirable steering wheel vibrations, commonly known as nibble and wheel fight, may be caused by road surface artifacts such as potholes or washboard dirt road surfaces, and vehicle dynamic events such as braking (brake roughness and braking torque), tire/wheel imbalance, or similar forces caused by tire construction. Nibble is a problem to varying degrees in a wide variety of vehicles, and can occur while driving on smooth roads at steady state conditions. Nibble can lead to customer dissatisfaction and may cause premature deterioration of vehicular components.
Nibble in vehicle steering systems has been exacerbated by recent developments in rack and pinion steering gears and other measures taken to reduce friction in vehicle steering systems and thereby improve vehicle handling. Nibble has also been exacerbated by customers' desire for larger diameter wheels and tires, which create larger forces.
One way to reduce nibble is to dampen it by increasing friction in a vehicle's steering system. However, higher unmodulated friction in steering systems can lead to imprecise steering. Current nibble countermeasures employed by vehicle manufacturers, including tuned dampers, only dampen at predefined frequencies of vibration and therefore require very careful matching of damper-tuned resonance to the system resonance to be controlled. In addition, these countermeasures require balancing of a vehicle's wheel/tire assembly during production.
Nibble can occur over a range of frequencies (e.g., 10-20 Hz) that generally correlate with the rotational speed of the tires. Further, identical vehicles can have different levels of nibble response. Depending on the frequency and the amplitude, vibrations can be mild to annoying. Thus, nibble can cause a reduction in customer satisfaction and increased warranty costs.
Conventional damping devices used for nibble mitigation utilize tuned dampers that work by removing energy from the steering system in a frequency range to which they are tuned. These dampers can be located in the steering wheel or attached to the steering shaft. They often comprise inertial masses that are suspended by springs or elastomeric compounds, such as the device disclosed in U.S. Pat. No. 6,614,689. This type of device has some drawbacks. For example, tuned dampers are typically tuned to a single frequency and cannot remove energy at other frequencies. Tuned dampers may even increase vibration at other frequencies. Thus, for tuned nibble dampers, if the vibration frequency shifts or occurs out of the range of the tuned damper, the damper is no longer effective. Frequency shifts can occur due to vehicle aging.
In addition, tuned dampers add weight and inertia to a vehicle steering system, which has a detrimental effect on vehicle steering and handling. The effectiveness of tuned dampers is limited by the amount of mass that can be used due to packaging constraints (i.e., the amount of available space for the mass, based on the vehicle's design). Further, many tuned dampers are not very effective for small amplitude vibrations, such as those characteristic of nibble. Tuned dampers are less effective for small amplitude vibrations due to the viscoelastic response of the elastomers used in the springs of tuned dampers.
Some conventional steering wheel vibration suppression devices utilize hydraulic or viscosity control of fluids. These designs can be costly and complicated to manufacture and assemble into vehicles, and often are subject to the same drawbacks as tuned dampers.
Magnetorheological (MR) dampers enable electrical control of torque transfer and rotational slip, in addition to control of the level of static and dynamic frication between two fixed surfaces. MR dampers typically use MR fluids, which commonly comprise slurries of 2 to 5 micron particles suspended in oil or other suitable carriers, such as water. As magnetic fields are applied to MR fluid, the particles tend to form chains that are capable of carrying torque proportional to the magnetic field.
Some disclosed MR damper configurations utilize MR fluid that is retained in the device with seals. The sealing of fluids can be costly and prone to leakage.